Light emitting diodes (LEDs) are key components to a wide range of applications that include backlighting units for liquid crystal displays, headlamps for automobiles, or general lighting. For example, nitride semiconductor based blue and green emitting LEDs are widely used in these applications. However, such LEDs still suffer from degraded performance at high current injection caused by a phenomenon commonly referred to in the art as “efficiency droop”.
It has been reported in the art that one possible cause of the efficiency droop may be due to the presence of an internal polarisation field in nitride LEDs. Indeed, the crystal layers in group III-nitride devices are often grown as strained wurzite crystals on lattice-mismatched substrates such as sapphire. Such crystals exhibit two types of polarisations: spontaneous polarisation, which arises from the crystal symmetry, and piezoelectric polarisation, which arises from strain. The total polarisation is the sum of the spontaneous and piezoelectric polarisations.
Such internal polarisation field has potential detrimental effects on the internal quantum efficiency of nitride LEDs. For example, the polarisation field across an InGaN quantum wells active region leads to band bending, which makes it more likely that electrons jump across quantum wells and escape into p-type layers. This band bending may also limit the holes injection efficiency into the quantum wells by reducing the hole mobility at the interface of two group III-nitride layers of different composition. In addition, the internal polarisation field affects the radiative recombination efficiency by separating the electron and hole wave-functions. More details about the efficiency droop mechanism can be found in the following articles and in their references: J. Piprek, Phys. Status Solidi A, 1-9 (2010), C. S. Xia et al., Appl. Phys. Lett. 99, 233501 (2011); and Y. K. Kuo et al., Appl. Phys. Lett. 95, 011116 (2009).
Therefore, it is desirable to reduce the effect of the internal polarisation fields on the carrier diffusion and carrier recombination, so the light output power of group III-nitride LEDs is improved.
An approach for reducing or cancelling the effect of the internal polarisation field is to grade the composition of the quantum well layers to generate space charges and quasi-fields that oppose polarisation-induced charges. This approach is described in Ibbetson et al., U.S. Pat. No. 6,515,313, issued on Feb. 4, 2003. Ibbetson discloses that a continuous or discrete grading of the indium concentration in the active region creates a quasi-field opposite to the internal polarisation field, thus leading to a better spatial overlap of the carriers increasing emission efficiency.
A second approach, described in Shen et al., U.S. Pat. No. 7,122,839, issued on Oct. 17, 2006, includes grading the composition of either the quantum wells or the barriers, such that the change in the composition is at least 0.2% per angstrom.
A third approach, described in Kim et al., U.S. Pat. No. 7,902,544, issued on Mar. 8, 2011, includes using quantum barrier layers featuring a band-gap modulated multilayer structure, instead of grading the composition like in the previous two patents. The multilayer structure includes at least two types of layers with different band gaps, the two types of layers stacked repeatedly. The consequence of using such a structure is a stronger current spreading effect.
However, none of the patents referenced above describe how to control the composition profile in each of the barrier layers in the case of an LED active region that has more than two quantum wells.
Another approach, described in Lee et al., U.S. Pat. No. 7,649,195, issued on Jan. 19, 2010, includes at least one middle barrier layer having a band gap relatively wider than the other barrier layers in the active region of a multiple quantum well LED structure. Accordingly, positions at which electrons and holes are combined in the multiple quantum well structure to emit light can be controlled, and luminous efficiency can be enhanced. However, this structure does not reduce the internal polarisation filed effect, and the carriers are still affected by this field.